The iron-nitrogen system: the preparation and the crystal structures of nitrogen-austenite( γ ) and nitrogen-martensite( α' )

Abstract

The nitrogen atoms of nitrogen-austenite occupy up to a maximum of about one in ten of the octahedral interstices of the face-centred cubic arrangement of iron atoms in a completely random manner. In nitrogen-martensite the same proportion of the octahedral interstices at ½, ½, 0 and 0, 0, ½ of the body-centred tetragonal iron-atom lattice is randomly occupied. Nitrogen-austenite and nitrogen-martensite are therefore isostructural with the corresponding iron-carbon alloys. The observed unit-cell dimensions vary with interstitial-atom concentration in the same way as for carbon-austenite and carbon-martensite, and for a given concentration are no smaller than in the latter alloys in spite of the smaller size usually assumed for the nitrogen atom. The displacements of iron atoms from the lattice points of martensite, due to the presence of nitrogen atoms are considerably smaller than those for carbon-martensite calculated from X-ray data given by previous workers. The values now found for nitrogen-martensite are consistent with the observed cell dimensions and with a nitrogen-atom radius of about 0⋅7 Å. A modified interpretation of the martensite structure accounts for all the X-ray diffraction data.